The present invention relates to a projection type liquid crystal display unit which exhibits a high quality display image and, more particularly, to an optical arrangement around a liquid crystal cell in such a unit.
In recent years, projection type display units have received attention as such devices can provide a large size display relatively easily. In particular, projection type liquid crystal display units, which make use of a liquid crystal panel as a light valve are commercially available, with a compact and bright display. FIG. 13 schematically shows an optical system used in a prior art front projection type liquid crystal display unit. White light produced by a light source lamp 51 is collected by means of a light collecting mirror 52 and is divided by a pair of dichroic mirrors 53, 54 into three components: namely, blue light (B) 55 having wavelengths of 400 to 510 nm, green light (G) 56 having wavelengths of 490 to 580 nm and red light (R) 57 having wavelengths of 570 to 700 nm. These light beams of three colors are made to impinge upon liquid crystal display panels 58, 59 and 60 for blue, green and red colors, respectively, so that the light intensities of the light beams of the respective colors are spatially modulated. The light beams emanating from these panels are integrated through a pair of dichroic mirrors 61, 62 to form a composite light beam which is projected on a front screen by means of a projection lens 63. It is thus possible to obtain a color display of a large size. A liquid crystal driving circuit capable of controlling the light intensity modulation is connected to each of the liquid crystal display panels, so that the display is controlled in accordance with externally supplied electrical signals. In general, the liquid crystal panel incorporated in this kind of projection type liquid crystal display unit is a twist nematic liquid crystal panel (hereafter referred to as "TN liquid crystal panel") of active-matrix type incorporating thin-film transistors. FIG. 14 shows in section an example of such a TN liquid crystal panel of active matrix type. As will be seen from this Figure, the liquid crystal display panel has a liquid crystal cell 1 and a pair of polarizing plates 2a, 2b which are arranged on both sides of the liquid crystal cell 1, the liquid crystal cell 1 having an array substrate 5a carrying a matrix of several tens to several hundreds of thousands of pixel electrodes 7, a film transistor 9, an opposing substrate 5b and a liquid crystal layer 6 formed between these substrates. The inner surfaces of the substrates have been orientation-treated, for example, by rubbing, such that both substrates give orientation in directions which substantially orthogonally cross each other. As a result, the liquid crystal molecules in the vicinity of the substrates are arranged such that their longer axes are directed in conformity with the orientation directions, so that the molecules in the liquid crystal layer are arranged in a twisted condition. Any light which is transmitted through the liquid crystal cell is rotated due to double refraction characteristics of the liquid crystal molecules and the twisted arrangement of the liquid crystal molecules. This phenomenon will be referred to hereafter as "optical rotation". When a voltage is applied to the liquid crystal cell, an electric field is generated in the thicknesswise direction of the liquid crystal layer so that the liquid crystal molecules are rearranged such that molecule axes of the liquid crystal molecules rise up and extend in the direction of the electric field, due to dielectric anisotropy of the liquid crystal molecules. As a result, the twist of the liquid crystal molecules and, hence, the optical rotation are extinguished. It is therefore possible to control the quantity of light passing through the liquid crystal panel by varying the voltage applied to the liquid crystal cell through a pair of polarizing plates arranged on both sides of the liquid crystal cell. When the two polarizing plates are arranged with their transmission axes extending in parallel with each other, a mode called "normally black mode" (hereafter referred to as "NB mode", ) is obtained in which the display becomes dark when no voltage is applied, whereas, when the arrangement is such that the transmission axes of these two polarizing plates are orthogonal to each other, a mode called "normally white mode" (referred to as "NW" mode, hereinafter) is obtained. The NB mode is preferably used as the liquid crystal panel of a projection type liquid crystal device because this mode presents a greater ratio of opening and, hence, a brighter display.
The optical rotary power in the liquid crystal layer varies according to the wavelength, so that the transmittance of the liquid crystal panel varies depending on the wavelength. This phenomenon will be referred to a optically rotatory dispersion. The liquid crystal panels for R, G and B colors, which receive light beams of different wavelengths, are required to have optical characteristics which adapt to the respective wavelength regions. Therefore, in order to conduct a dark display or a display with a certain degree of contrast in the NB mode it is necessary that the liquid crystal panels for B, G and R colors have thicknesses which are determined in accordance with the wavelength regions of the respective colors or that the angles at which the optical axes of the polarizing plates are set are determined in accordance with the wavelength regions of the respective colors. FIG. 15 shows spectral transmission characteristics of B, G and R panels having different thicknesses of the liquid crystal layer, as observed when these panels are in a dark display state. The panel for B color interrupts light of wavelengths around 460 nm, while panels for G and R colors interrupt light of wavelengths around 540 nm and 610 nm, respectively. The wavelength regions of light interrupted, however, are narrow due to optically rotatory dispersion.
FIG. 16 shows intensity distributions of the light impinging upon these panels. These light beams have been obtained by separation through dichroic mirrors and have wavelength region widths of about 100 nm. Such widths are necessary for obtaining a bright display. The panels shown in FIG. 15, therefore, cannot satisfactorily interrupt the light shown in FIG. 16, so that a display with high contrast and display of pure black color cannot be conducted with the composite light composed of light of these three color. The levels of contrast obtained in the above-described display unit are shown below.
______________________________________ Panels B G R ______________________________________ Contrast levels 40 80 95 ______________________________________
It is also to be pointed out that, in a projection type liquid crystal display unit which employs a high-power light source, the temperature of the whole unit and of the liquid crystal display panel is gradually raised to a level about 20.degree. C. higher than the room temperature so as to cause a change in the spectral characteristic of the liquid crystal panel. Thus, the wavelength of minimum transmittance in each liquid crystal panel is shifted to the shorter wavelength side in amount of about 20 nm. As a result, the deviation of the optical characteristics of the liquid crystal panel from the color light of intensity distribution shown in FIG. 16 is increased, thus further degrading the quality of the display image. Furthermore, the liquid crystal display unit of the type described necessitates liquid crystal cells of different thicknesses for different colors and, hence, a complicated production process is required which hampers production.
The liquid crystal display unit of the second-mentioned type, i.e., the unit in which the display panels for B, G and R colors have different set angles of polarizing plates, exhibits light interrupting characteristics of even greater inferiority to those shown in FIG. 15, so that the quality of the display image is inferior even in comparison with that of the first-mentioned type of liquid crystal display unit.
In order to obviate the above-described problems, the specification of Japanese Patent Unexamined Publication No. 1-277282 proposes a method in which the liquid crystal panel for each color is provided with a compensation liquid crystal cell which has the same value of the product .DELTA.nd of the double refraction index .DELTA.n and the liquid crystal layer thickness d (.DELTA.nd=.DELTA.nxd) as the color liquid crystal panel and a twisting direction opposite to that of the color liquid crystal display panel. This method makes it possible to obtain a display image of a high contrast, as well as a pure black display. Unfortunately, however, this method causes about 30% reduction in the brightness, making it difficult to obtain a display having a high level of brightness. Furthermore, this method undesirably raises the production cost, because the number of the liquid crystal cells employed is doubled.
Thus, the known projection type liquid crystal display units and display method have suffered from problems such as incompatibility between bright level of display and ability to conduct high-contrast and pure black display, degradation of the display image quality due to a temperature change, and high production cost.